One of the features of this invention is to provide a pressure fluid engine comprising a rotatable rotor, a casing in which the rotor is enclosed and peripherally spaced from the rotor a distance sufficient to prevent the exhaust fluid within the casing from operating as a fluid friction drag on the rotating rotor and with the fluid entrance to the rotor having an area greater than the fluid exit from the rotor so as to achieve efficient conversion of fluid pressure energy to mechanical rotational energy of the rotor by mechanical conversion energy means in the rotor.